Eurail pass

It is generally cheaper to buy point-to-point tix there (usually from Machines that have an English language option). As for passes not being accepted, that would be true on special lines that are privately owned and likely on Thalys, the high-speed train between Paris and Brussels (and, we now think, Amsterdam). Have never heard of passes being rejected on SCNF trains or on Deutsche Bahn. Just buy the tix there-- it is easy and relatively cheap

I have never encountered this problem. Were they traveling in a country where the pass wasn't valid?

It depends where and the distances you will be traveling to determine whether the pass would be a bargain for you. If you also don't want the hassle of buying tickets at the station all the time, a pass would be easier. If you are traveling short distances a pass would be the more expensive route.

SNCF (French Rail) does limit the number of seats it allocates to passholders on TGVs. If those seats are gone on a particular train, a person would have to buy a regular ticket--or take a different train.

You can find point-point ticket prices from the national rail companies websites. You can also find the extra costs (passholder supplements) for rail passes on the Eurail website (www.eurail.com). Add up the P-P prices and compare them to the cost of a pass plus passholder supplements. Unless the railpass offers significant savings, it might not be worth it. It is true that often passholder tickets are limited in quantity. I have heard horror stories of passholder stuck in Brussels in peak times because passholder tickets on Thalys were sold out for days.